Metaphase and anaphase in the artificially induced monopolar spindle

By using monopolar spindles artificially induced in sea urchin embryos, we examined whether or not the presence of two opposing poles was an indispensable condition for keeping chromosomes at a fixed distance from the pole at metaphase and for the anaphase chromosome movement. Chromosomes were stained with Hoechst dye 33342 and their behavior was followed in the monopolar and the control bipolar spindles. In the monopolar spindle, chromosomes were first arranged on a curved metaphase plate and then spread on a part of the imaginary surface of a sphere whose center was the monopole. The estimated chromosome-to-pole distance was similar to that of bipolar spindles at metaphase and remained fixed until chromosomes started to move toward the pole. The average duration of metaphase in the monopolar spindle was 6 times longer than that in the bipolar spindle. The poleward movement of chromosomes in the monopolar spindle was similar to the anaphase A (chromosome-to-pole movement) in the bipolar spindle with respect to the velocity, duration, distance, and synchronization of migration. These results show that even half of the normal spindle has capacities for the arrangement of chromosomes at metaphase and for the anaphase A chromosome movement. Based on these results, we were able to exclude some existing theories of metaphase, such as the one based on the balance of forces between the two poles.